1. Field of the Invention
The present invention relates generally to vehicle air conditioners, and more particularly to an air conditioner with an inside heat exchanger (or evaporator) that is controlled to operate either in a heating or a cooling mode, and that is controlled so that water condensed thereon is prevented from fogging the inside of a vehicle windshield at the start of the heating mode.
2. Related Art
In a conventional vehicle air conditioner, hot water (or engine cooling water) is circulated in a heating heat exchanger during cold weather so that air to be conditioned may be heated via the hot water. In this case, when the hot water temperature is low, the temperature of air to be conditioned may be insufficient for vehicle compartment heating purposes.
Japanese Patent Application Hei. 5-272817, describes a refrigerating cycle system having a hot gas heater cycle heating function. When the hot water temperature is lower than a predetermined value as at the engine starting time, the above system causes compressor discharged gas coolant (or hot gas) to be introduced into an evaporator while bypassing a condenser to release the heat from the gas coolant to the air to be conditioned in the evaporator. In the above system, more specifically, the evaporator, or a common inside heat exchanger disposed in an air conditioner duct, is selectively used as a cooler in the cooling mode and a radiator in the heating mode.
In the air conditioner discussed above, an inside air mode may be set for preventing contaminated outside air from being introduced during cold weather operation of the heating mode. In this case, it is necessary for the evaporator to both cool and dehumidify the air to be conditioned to prevent the windshield from fogging. Until the ambient air drops to 0.degree. C., therefore, the refrigerating cycle may be operated in the cooling mode.
In the case, the temperature of the evaporator fluctuates greatly as a result of the intermittent compressor operation. Generally, when the ambient temperature drops to or below 0.degree. C., the compressor is stopped. When a variable displacement type compressor is used, however, the evaporator temperature can be more stably controlled with less evaporator temperature fluctuation by controlling the low cycle pressure (or coolant evaporation pressure) via compressor displacement. Therefore, the compressor is operated so that the evaporator operates in a cooling/dehumidifying mode even when the ambient temperature is about -10.degree. C.
On the other hand, the hot gas heater cycle performs an auxiliary heating function in addition to the main heating function of the hot water type heating heat exchanger and is used in an ambient temperature range as low as about +10.degree. C.
Just after the refrigerating cycle is used at an ambient temperature of about 0.degree. C. in the cooling mode for preventing the fogging of the windshield, therefore, the mode may be switched to the heating mode due to insufficient heating capacity at the hot water heating heat exchanger.
When the hot gas heater cycle is switched to a heating mode, however, the evaporator, having just operated in the cooling and dehumidifying modes, is changed all at once to the hot gas heating mode so that the temperature of the evaporator abruptly rises. Therefore, water that condensed on the surface of the evaporator at the time of the cooling mode evaporates again, and is carried by the conditioned air into the vehicle passenger compartment, where it causes the inside of the vehicle windshield to fog.
The condensed water once generated in the evaporator during the cooling mode operation does not easily evaporate at low ambient temperature in winter and may remain for a long time. Even just after switching from the cooling mode (that is, even if the cooling mode is not used for a long time), therefore, the vehicle windshield may fog at the start of the hot gas heater cycle heating mode.